OrcaFlex Manual - Orcina

VIV Toolbox, Time Domain Models
470
w
� It does not use sheet detachment to separate the vortices coming from one side of the disc into separate sheets.
� It does not use entrainment, nor rediscretisation to keep the vortices at equally spaced arc lengths along the
sheet.
� It uses a coalescing algorithm to control the number of vortices.
Coalescing
To keep down the number of vortices being tracked, model 2 tries, at each time step, to coalesce pairs of vortices
that have come very close to each other. This is done as follows:
� At each time step the model finds, for each vortex, the nearest neighbouring vortex. If two vortices are each
other's nearest neighbours then they are called 'mutually nearest neighbours', and such a pair are considered
for possible coalescing into one.
� Such a pair of mutually nearest neighbours are coalesced if their separation is less than Sc, where Sc is a
coalescing separation that depends on the distance D from the two vortices to the disc surface. Sc is given by:
Sc = λ.R if D≤2R
Sc = λ.R.(D/2R) 2 if D>2R
where R is the disc radius and λ is the coalescing threshold specified by the user. The threshold used depends on
whether the two vortices are of the same or opposite signs.
� These formulae for Sc mean that the user's data sets, in disc radius units, the coalescing separation for vortices
that are within 2 disc radii of the disc surface, and for vortices further away the coalescing separation increases
according to the square of the distance from the disc surface. The aim of this is to restrict coalescing near to the
disc but encourage it once the vortices have convected significantly away from the disc.
� If the two vortices are coalesced then they are replaced by one vortex whose strength is the sum of their signed
strengths and which is placed at their centroid of absolute vorticity.
Data and Results
Data
The following data needs setting for the vortex tracking models. See also the data that is common to all the time
domain VIV models.
Maximum Number of Vortices Logged (per side of the line)
This controls the maximum number of vortices that will be displayed and logged, for each side of the line. Note that
this setting only affects the display of vortices and does not affect the calculation itself in any way. Its purpose is
only to allow you to control the size of the log file (and hence the simulation file) and to control the speed of drawing
the 3D view. You can set the maximum number of vortices to one of:
� 0 is the default value, meaning 'log and draw no vortices'. The log file and simulation file are then as small as
possible and the replay as fast as possible. However the drawback of doing this is that you will not see any
vortices on the 3D view.
� '~', meaning 'log and draw all vortices'. There are typically up to several hundred vortices generated per node,
and each vortex needs to log its position and strength so that it can be drawn on the 3D view. So if all vortices
are logged then the log file and simulation file can therefore be quite large. Also the replay might be slower
because many vortices must be drawn for each frame of the replay.
� An intermediate value, 30 say, allows you to see the youngest 30 vortices per side of the line. Note that if you do
this then as new vortices are generated at the separation points you will see the oldest vortices (typically now
downstream) disappear from view. They have not been destroyed and will still be in the calculation, but their
position and strength is no longer available to the 3D view.
Model Parameters
If the Default option is chosen then appropriate default values for the model parameters are used. Alternatively the
Specified option can be selected which gives you complete control over the model parameters.
Warning: The Specified option has been provided principally to allow users the option of calibrating the
model against other experimental results. If you are not doing this then we strongly recommend
that you use the Default parameters.